Coronary heart disease remains the leading cause of morbidity and mortality in the United States and the developed world. Although the current “gold standard” for assessing coronary artery disease (CAD) is angiography, it has serious limitations in evaluating the functional significance of intermediate coronary lesions (comprising 30-70% stenosis). Coronary angiography relies on a visual interpretation of coronary anatomy. A number of studies have documented the large intra- and inter-observer variability that results from visual grading of coronary stenotic lesions. Moreover, studies have shown a lack of correlation between the angiographic delineated stenosis with their physiologic severity on coronary flow. This stems from the highly non-linear relation between the degree of stenosis and the change in blood flow. Typically, the blood flow remains unchanged until the degree of stenosis reaches a critical range (typically >80%), at which point the decrease in flow is quite dramatic. Lesions that are not functionally significant (i.e., do not reduce the flow) may not need treatment. Hence, there is a need for complementary methods to conventional coronary arteriograms that combine coronary anatomy and physiology to assess CAD accurately.
Blood vessel diameter or cross-sectional area gives anatomic measures of stenosis severity. Coronary blood flow, on the other hand, reflects coronary hemodynamic function and can be used to assess functional severity of stenosis through parameters such as coronary flow reserve (CFR) and fractional flow reserve (FFR). CFR, defined as the ratio of hyperemic (induced by pharmacological agents) to resting flow in a coronary artery. It has been previously found that a significant stenosis leading to inducible ischemia occurs when CFR has a value less than 2.0. Normally, the coronary circulation has a flow reserve of 3-5 times that of normal resting blood flow. This reserve stems from the tone of small blood vessels (microvascular bed). In disease, the microvascular bed dilates and uses some of its reserve to compensate for the pressure drop to the stenosis. Hence, a low CFR value can characterize disease in the epicardial arteries or the distal resistive microvascular bed.
CFR can be estimated from hyperemic and resting blood velocities measured by a Doppler guidewire. This method is based on the principle of Doppler which requires that the piezo-electric crystal to be at a specific angle to the flowing blood. Since this condition is very difficult to meet in clinical practice as the tip of the wire is difficult to align with the direction of flow, the measurements are not reliably accurate and this method has not enjoyed clinical utility. Recent developments have introduced methods and systems for accurate determination of cross-sectional area of blood vessels including coronary arteries. Simultaneous measurements of cross-sectional area and flow (including CFR) would provide a clinician with a greater insight in the contribution of the epicardial vessel and microvasculature to total resistance to myocardial blood flow.
In summary, there are well-known limitations to the use of visual estimation to assess the severity of coronary artery disease and luminal stenosis. This is especially true in the case of intermediate coronary lesion where coronary angiography is very limited in distinguishing ischemia-producing intermediate coronary lesions from non-ischemia-producing ones. For this reason, a functional measure of stenosis severity is desirable. Previous devices involving Doppler flow wires also have serious limitations as referenced above. Hence, there is clearly a need for a simple, accurate, cost effective solution to determination of coronary blood flow in routine practice.